1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to a method of estimating the location of a terminal in a switched-beamforming based wireless communication system.
2. Description of the Related Art
In conventional cellular wireless communication systems, an access point (AP) transmits and receives signals using an omni-directional antenna, and according to the characteristics of the omni-directional antenna, the AP can communicate with terminals in a service area but cannot communicate with terminals outside the service area.
An IEEE 802.11 based wireless local area network (WLAN) is one of the cellular wireless communication systems. FIG. 1 is a schematic configuration of a conventional wireless communication system. Referring to FIG. 1, a service area 10 of an AP 110 is determined by the level of transmission power. Terminals 121 and 122 in the service area 10 can communicate with the AP 110, but a terminal 123 outside the service area 10 cannot communicate with the AP 110.
FIG. 2 is a schematic configuration of a WLAN operated by an improved AP 210. Referring to FIG. 2, the AP 210 communicates with all terminals using a smart antenna. The AP 210 having the smart antenna can form a beam directing each of the terminals, and each of the terminals also can form a beam directing the AP 210, thereby improving system performance. If an IEEE 802.11 system uses the smart antenna, since the AP 210 can concentrate transmission power in a specific direction in which a terminal is located, as illustrated in FIG. 2, a service area can be expanded in the specific direction.
However, according to the characteristics of a carrier sense multiple access (CSMA) mechanism of the IEEE 802.11 system, a certain terminal examines whether other terminals occupy a channel and operates according to the channel state. Thus, when a directional beam is used, the certain terminal cannot detect signals transmitted from other terminals in the WLAN. In other words, a terminal of the IEEE 802.11 system measures signals transmitted from APs and other terminals, and if signals transmitted from the other terminals are not detected, the terminal tries channel occupation. Thus, when a directional antenna is used, in many cases, a terminal determines that a channel is in an idle state even if the channel is being used. This phenomenon, called a hidden beam problem, significantly affects the system performance.
In addition, in a smart antenna mechanism, the movement of a terminal must be traced for reliable communication, and a direction of arrival (DOA) scheme is used in general as a terminal location estimation scheme.
FIG. 3 is a conceptual diagram for explaining the DOA scheme in a conventional cellular system using a smart antenna. Referring to FIG. 3, when a terminal 320 communicating with an AP 310 in a beam area 30 moves, the AP 310 cannot detect the movement of the terminal 320. Thus, the terminal 320 transmits a beam request message to the AP 310 to inform the AP 310 of a location thereof, and then, the AP 310 can detect a location of the moved terminal 320′ through a DOA algorithm by receiving the beam request message. When the AP 310 detects the location of the terminal 320′, the AP 310 communicates with the terminal 320′ by forming a beam directed to the detected location.
However, in the conventional DOA scheme, since an AP cannot detect the movement of a terminal until the AP receives a beam request message from the terminal, the probability that a downstream link transmission error is generated is high. In addition, since the AP must perform the DOA algorithm every time the AP receives a beam request message from the terminal, an excessive processing overhead is generated.